The air mattress formed of cells inflated by air pressure has been conventionally known. The air mattress is used as a Body pressure dispersion mattress and the like, and this mattress is useful for presenting bedsores from occurring over the body of a patient by dispersing the load of the body of the patient lying over the mattress to reduce the pressure acting on the body.
Herein, for individual cells of the air mattress, it is necessary to regulate the pressure inside each cell within a prescribed range (which will be referred to hereinbelow as “internal pressure control”). In order to perform internal pressure control, the pressure is measured first using a pressure sensor, then based on the measured pressure, each cell is inflated or deflated to regulate the pressure.
Herein, before performing internal pressure control, zero-point adjustment should be performed in order to measure pressure correctly. As a method for performing zero-point adjustment, a device disclosed in Patent Document 1 has been known, for instance.